1. Field of the Invention
The invention relates to a process for converting distillation resides obtained from polymerization processes to useful products. More specifically, the process is directed to the maleation of distillation residues to produce maleated products characterized by having viscosities which are shear dependent.
2. Description of the Prior Art
High density polyethylene (HDPE) resins are widely utilized for film applications such as for grocery sacks, merchandise bags, can liners and the like. HDPEs are typically produced by copolymerizing ethylene with a minor amount of a C3-8 α-olefin comonomer using either Ziegler-Natta catalysts or the so-called Phillips catalysts. The latter are chromium oxide catalysts and generally produce HDPE resins having broad molecular weight distributions (MWDs) whereas Ziegler-Natta catalysts, which are based on transition metal technology, produce narrower MWD HDPEs.
While most HDPEs exhibit good tensile and stiffness properties, certain improvements, such as increased tear properties and increased impact strength, can be achieved by increasing the molecular weight of the resin. High molecular weight resins are, however, more difficult to process for film production and require the use of higher processing temperatures and/or pressures. To ameliorate this effect, high molecular weight high density polyethylene (HMW HDPE) film grade resins preferably have broad MWDs.
Multiple-stage polymerization technologies wherein polymers of different molecular weights are produced in separate reactors and blended to produce a final resin product are a known means of producing resins having broadened MWDs (see e.g., U.S. Pat. No. 5,236,998)
U.S. Pat. No. 4,357,448 discloses a process wherein ethylene or a mixture of ethylene and a small amount of another α-olefin are polymerized in two successive steps under different hydrogen partial pressures using high activity Ziegler-type catalysts to produce HDPE resins having broad MWDs. A similar procedure for the production of high molecular weight medium density polyethylene resins is disclosed in U.S. Pat. No. 6,770,715.
In one mode of operation for the production of HMW HDPEs where successive polymerization steps are employed, ethylene is homopolymerized in a first reactor in a hydrocarbon diluent, such as hexane or heptane, and the amount of molecular weight regulator, i.e., hydrogen, is maintained at low levels to maximize molecular weight of the homopolymer formed. The high molecular weight homopolymer produced in the first reaction zone is then fed along with the solvent and catalyst to a second reaction zone where ethylene and a C3-8 α-olefin comonomer are copolymerized in the presence of the homopolymer. The ratio of homopolymer produced in the first reactor to copolymer produced in the second reactor (which typically has a lower molecular weight) is selected to provide the desired average molecular weight and MWD in the final resin product for optimal physical properties and processing characteristics.
While such processes are an effective and versatile means for producing a broad array of HMW HDPE resins of varying densities and melt indexes (MIs), substantial amounts of low molecular weight polymers (LMWPs) are also formed. The LMWPs have number average molecular weights (Mn) from several hundred up to about 30000 and, more typically, up to about 20000. These low molecular weight by-product polymers have a waxy character and they remain in the hydrocarbon diluent after separation and recovery of the HMW HDPE by centrifugation or other suitable means.
In a typical HMW HDPE operation, the hydrocarbon diluent containing the LMWP, any unrecovered HMW HDPE and catalyst residue is subjected to one or more distillations to recover the hydrocarbon which is recycled for reuse in the polymerization process. The still “bottoms” obtained from the distillation, also referred to herein as the polymerizer/polymerization residue or by-product, generally contain about 60 to 90 weight percent (wt. %) LMWP, 5 to 20 wt. % HMW HDPE, 4 to 10 wt. % hydrocarbon diluent and 0.2 to 1 wt. % catalyst residue and catalyst deactivating agents, e.g., alcohols.
Even though the low molecular weight ethylene polymer waxes are the major constituents, these polymerizer residues cannot be used as such for most wax applications due to the presence of significant levels of the high molecular species (which increase the viscosity to a level outside the useful range for most wax applications) and their high metals content due to the presence of catalyst residues (which form undesirable color bodies).
Since separation of the low and high molecular weight species and removal of catalyst residues is difficult and not economically feasible, it would be highly desirable if a process were available whereby the polymerization residues recovered from such processes could be effectively treated and converted into useful products. These and other advantages are achieved with the process of the present invention which is described in detail to follow.